1. Field of the Invention
The present invention relates to an apparatus for supporting an optical system, such as an objective lens system for converging a light beam emitted from a light source and making it incident upon an optical recording medium such, as an optical disc, in a focusing condition; the apparatus is for use in an optical information recording/reproducing apparatus.
2. Related Art Statement
Such optical pick up device is required to be made compact in size and thin in thickness. In order to make the thickness of the optical pick up device thin, such an arrangement is suggested that a mirror is arranged just under an objective lens driving means, and an optical path of a light beam being made incident upon the objective lens is changed by 90.degree. toward the objective lens. However, in this case, it is impossible to make the thickness of the pick up device thinner than the total heights of the objective lens driving means and the mirror arranged under the objective lens driving means.
In order to make the thickness of the optical pick up device thinner, in Japanese Utility Model Preliminarily Publication No. 56-77028, an optical pick up device having such an arrangement is disclosed that an opening is formed in a center portion of a holding member for holding an objective lens; a mirror for reflecting a light beam is arranged in the opening; coils and magnetic circuits for driving the objective lens in a focusing direction are provided under the mirror; and supporting members for supporting the objective lens driving means are arranged in upper and lower sides of the mirror.
However, in such arrangement disclosed in Japanese Utility Model Preliminarily Publication No. 56-77028, since the mirror is arranged between the upper and lower supporting members, the distance between the upper supporting members and the lower supporting members should be made wider than a diameter of light bundle of the incident light beam. Further, since the lower supporting members, the coils and the magnetic circuits are arranged under the incident light beam, it is difficult to make the thickness of the optical pick up device as a whole thinner.
In order to correct this problem, the present inventors suggest a novel apparatus for supporting an optical system in Japanese Patent Preliminary Publication No. 2-226523. FIGS. 1 and 2 are schematic views showing a construction of the apparatus disclosed in the publication. As shown in FIG. 1, the mirror 101, by which the light beam emitted from an optical unit 102 is reflected toward an objective lens 104, is arranged just under a holding member 103 for holding the objective lens 104. The holding member 103 is arranged to be movable in Z direction with the aid of springs 105a, 105b, 106a and 106b, which are extended in Y direction. Further, in this apparatus, a distance between the springs 106a and 106b, which are arranged in an outer side of an optical disc cartridge 107, is designed to be smaller than the distance between the springs 105a and 105b, which are arranged in an inner side of the cartridge 107, so the distance between the objective lens 4 and the mirror 101 can be made small. In such arrangement, it is possible to make the thickness of the optical pick up device small.
Further, in Japanese Patent Preliminary Publication No. 2-226523, a modification as shown in FIG. 2 is also disclosed. In this modification, the objective lens 104 is arranged to be movable only in Z direction; and the thickness of the holding member 103 is designed to be much thinner by arranging the mirror 101 in a concave portion of the holding member 103, i.e. just under the objective lens 104. In this modification, the objective lens 104 is arranged to be movable only in Z direction with the aid of springs 105a, 105b and 106. It should be noted that only one spring 106 is arranged in the outer side of the cartridge 107 and the position in height of the spring 106 is almost the same as that of the objective lens 104.
However, in the conventional apparatus shown in FIG. 1, since the center of gravity of a movable member comprising the objective lens 104 and the holding member 103 is at a point G, and all of the springs 105a, 105b, 106a and 106b cannot be arranged in an upper side of the center of gravity. Therefore, the light bundle emitted from the optical unit 102, being made incident upon the mirror 101, cannot be placed in an upper side of the center of the gravity G of the movable member of the pick up device.
The other conventional apparatus shown in FIG. 2 allows to make the thickness of the optical pick up device thinner, but the objective lens 104 could move only in Z direction, so that the arrangement shown in FIG. 2 could not be applied to the apparatus, which is required to move the objective lens in focusing and tracking directions. Further, in the apparatus shown in FIG. 2 the thicknesses (Z direction) and lengths (Y direction) of the three leaf springs 105a, 105b and 106 are arranged to be the same but the width (X direction) of the spring 106 is twice of those of the springs 105a and 105b. Therefore, two kinds of leaf springs each having different spring constant are required; and thus the number of parts becomes large. Furthermore, when processing these leaf springs, the spring constants are varied due to the variation in their manufacturing process. As a result, the holding member 103 is moved with some inclination and thus a proper movement of the holding member 103 could not be realized.